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Journal ArticleDOI

Diversity through semisynthesis: the chemistry and biological activity of semisynthetic epothilone derivatives

01 Jan 2011-Molecular Diversity (Springer Netherlands)-Vol. 15, Iss: 2, pp 383-399
TL;DR: The current review provides a comprehensive overview on the chemical transformations that have been investigated for the major epothilones A and B as starting materials, and it discusses the biological activity of the resulting products.
Abstract: Epothilones are myxobacterial natural products that inhibit human cancer cell growth through the stabil- ization of cellular microtubules (i.e., a "taxol-like" mech- anism of action). They have proven to be highly productive lead structures for anticancer drug discovery, with at least seven epothilone-type agents having entered clinical trials in humans over the last several years. SAR studies on epothil- ones have included a large number of fully synthetic ana- logs and semisynthetic derivatives. Previous reviews on the chemistryandbiologyofepothiloneshavemostlyfocusedon analogs thatwereobtainedbydenovochemical synthesis.In contrast, the current review provides a comprehensive over- view on the chemical transformations that have been investi- gatedforthemajorepothilonesAandBasstartingmaterials, and it discusses the biological activity of the resulting prod- ucts. Many semisynthetic epothilone derivatives have been found to exhibit potent effects on human cancer cell growth and several of these have been advanced to the stage of clini-

Summary (3 min read)

Introduction

  • MSA can be divided into two distinct functional classes, namely compounds that inhibit the assembly of soluble tubulin into microtubule polymers ("tubulin polymerization inhibitors") and those that promote the assembly of tubulin heterodimers into microtubule polymers and stabilize microtubules ("microtubule stabilizers") [4] .
  • After the elucidation of taxol's mode of action in 1979 [6] , it took more than a decade before other microtubulestabilizing agents with non-taxol-like structures were discovered.
  • Even for the natural product taxol [38] , the sustained supply of sufficient quantities of drug material for clinical use could only be secured for some time through the development of a semisynthetic production process from another natural product, namely 10-deacetylbaccatin III [39, 40] .
  • This will facilitate the comparison of the biological effects of related structural changes.

Modifications of the epoxide moiety

  • Modifications of the epoxide moiety have been an important trait of the semisynthetic work on epothilones from the very beginning, which is unsurprising in light of the multitude of transformations that are conceivable for an oxirane ring and the potential for further elaboration of the initial reaction products.
  • In contrast to the above acetonides, cis and trans diols 9 and 5 did not show any appreciable biological activity (IC 50 's for cancer cell growth inhibition >1 µM) [42, 43] .
  • As illustrated in Scheme 4, 12 was obtained through nucleophilic ring-opening of the epoxide moiety in Epo A with azide anion (to produce 11) and subsequent reduction of the azide group under Staudinger conditions (Ph 3 P/H 2 O) [48] .
  • These findings re-confirmed the notion that the oxirane ring system in epothilones merely serves to stabilize the proper bioactive conformation of the macrocyclic skeleton rather than acting as a reactive electrophile or a hydrogen bond acceptor.
  • Several of these derivatives show antiproliferative activities that are comparable with or even superior to that of Epo A [45] .

Modifications of the ester moiety

  • One of the most obvious modifications of the epothilone scaffold is the hydrolysis of the ester bond to produce the corresponding seco acid.
  • This transformation has been achieved by treatment of Epo A with NaOH/MeOH, which gave seco acid 25 in 65% yield (Scheme 8); however, ester hydrolysis was also accompanied by the retro aldol cleavage of the C3-C4 bond, thus leading to 23% of the retro aldol product 26 (Scheme 8) [58] .
  • While the situation is more complex in vivo and the stability of Epo B is much higher in human than in rodent plasma, their findings on the reduced activity of epothilones in the presence of mouse plasma have led the BMS group to pursue lactambased epothilone analogs as metabolically more stable alternatives to the natural macrolactones [41, 59] .
  • Using this chemistry, the BMS group has also developed an efficient one-pot process for the conversion of Epo B into lactam derivative 34, which involves the above Pd(0)-catalyzed ring-opening reaction, reduction of the azide with trimethyl phosphine and macrolactam formation with EDCI/HOBt [59] ; this provides the desired lactam in 23% overall yield.
  • In spite of its limited effects against highly multidrug-resistant cell lines in vitro, intriguingly, the compound has also been found to be superior to Taxol in Taxol -resistant tumor models [62] .

Modifications in the C2-C8 region

  • Semisynthesis-based modifications in the C2-C8 region have involved transformations of all functional groups present in this sector of the epothilone structure, including the hydroxyl groups at C3 and C7 as well as the keto group at C5.
  • After TES protection of 37 and 38, to give 39 and 40, 1,4-addition of cyanide ion produced a ca. 1/1 mixture of 3S and 3R isomers, which were separated and then deprotected with acetic acid individually to provide 41 and 42 (as well as their corresponding 3R isomers).
  • The in vitro activity of analogs 37/41 and 38/42 is less than one order of magnitude lower than that of Epo A and B, respectively; for 38 and 42 their in vitro antiproliferative activity is thus comparable with taxol [44] .
  • No oxidation of the 7-OH group takes place under these conditions, but the oxidized product is obtained as its C7-methylthiomethyl (MTM) ether 44 (30% yield; Scheme 11).

Side chain modifications

  • The C15 side chain of epothilones has been targeted for semisynthesis in a number of different ways that include modifications (or replacement) of the heterocycle, the vinyl linker between the heterocycle and the macrolactone ring, or both.
  • Both Epo E and F have been elaborated into different C21-modified derivatives [66] and C20 substituents of limited size (but still larger than the natural methyl group) have been found compatible with potent antiproliferative activity; more bulky substituents result in a substantial loss in potency.
  • ABJ879 (55) has demonstrated potent antitumor activity in experimental animal models [69] , where it produced transient regressions and inhibition of tumor growth of slowgrowing (NCI H-596 lung adenocarcinomas, HT-29 colon tumors) as well as fast-growing, difficult-to-treat tumors (NCI H-460 large cell lung tumors).
  • In contrast, olefination reactions involving the C16 keto group proved to be highly problematic.
  • In order to enable the replacement of the thiazole ring by other aryl moieties, Höfle and co-workers have developed an alternative strategy for the construction of the aryl-vinyl part of the epothilone side chain from ketone 59 (Scheme 17).

Replacement of the C13-O16 segment

  • Semisynthetic epothilone analogs have also been prepared via intermediates that were obtained through the degradative removal of the entire C13-O16 segment (including the pendant side chain), which were then (re)elaborated into modified versions of the original structure.
  • Treatment of the crude keto aldehyde thus obtained with K 2 CO 3 led to facile elimination of the C1-C12 segment as the free carboxylic acid, which was converted to ester 77 with TMS-diazomethane.
  • Oxidation of 82 with NaClO 2 gave the mono-ester of a dicarboxylic acid, which could be coupled with amines 83 or 84, to give the C12-C13 amide-based analogs 85 and 86, respectively, after ester saponification, Yamaguchi-type macrolactonization and final deprotection with CF 3 COOH (Scheme 20) [41] .
  • Unfortunately, none of the derivatives 85-88 showed any significant tubulin-polymerizing or growth-inhibitory activity.

Conclusions

  • The work discussed in this review article has defined the chemistry associated with the epothilone molecular framework in significant detail.
  • Most importantly, these efforts have led to the discovery of three derivatives that have been advanced to clinical studies in humans.
  • Thus, notwithstanding the wealth of fully synthetic analogs that have been prepared over the last 15 years and many of which exhibit very attractive biological profiles (at least in vitro), semisynthesis so far has had the more profound impact on the clinical advancement of the epothilone class of microtubule stabilizers than total synthesis.
  • At the same time, it is clear that the potential of semisynthesis for the creation of new structurally unique epothilone analogs is far from being exhausted.
  • It is unclear at this point, whether those groups with a sufficient supply of natural epothilones (BMS, GBF (now Helmholtz Centre for Infection Research), Novartis) continue to be active in the area of epothilone semisynthesis.

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Diversity through semisynthesis
The chemistry and biological activity of semisynthetic
epothilone derivatives
Review Article
Author(s):
Altmann, Karl-Heinz; Gaugaz, Fabienne Z.; Schiess, Raphael
Publication date:
2011-05
Permanent link:
https://doi.org/10.3929/ethz-b-000035789
Rights / license:
In Copyright - Non-Commercial Use Permitted
Originally published in:
Molecular diversity 15(2), https://doi.org/10.1007/s11030-010-9291-0
This page was generated automatically upon download from the ETH Zurich Research Collection.
For more information, please consult the Terms of use.

Mol Divers (2011) 15:383–399
DOI 10.1007/s11030-010-9291-0
COMPREHENSIVE REVIEW
Diversity through semisynthesis: the chemistry and biological
activity of semisynthetic epothilone derivatives
Karl-Heinz Altmann · Fabienne Z. Gaugaz ·
Raphael Schiess
Received: 6 July 2010 / Accepted: 25 October 2010 / Published online: 1 January 2011
© Springer Science+Business Media B.V. 2010
Abstract Epothilones are myxobacterial natural products
that inhibit human cancer cell growth through the stabil-
ization of cellular microtubules (i.e., a “taxol-like” mech-
anism of action). They have proven to be highly productive
lead structures for anticancer drug discovery, with at least
seven epothilone-type agents having entered clinical trials in
humans over the last several years. SAR studies on epothil-
ones have included a large number of fully synthetic ana-
logs and semisynthetic derivatives. Previous reviews on the
chemistry and biology of epothilones have mostly focused on
analogs that were obtained by de novo chemical synthesis. In
contrast, the current review provides a comprehensive over-
view on the chemical transformations that have been investi-
gated for the major epothilones A and B as starting materials,
and it discusses the biological activity of the resulting prod-
ucts. Many semisynthetic epothilone derivatives have been
found to exhibit potent effects on human cancer cell growth
and several of these have been advanced to the stage of clini-
cal development. This includes the epothilone B lactam ixab-
epilone (Ixempra
), which has been approved by the FDA
for the treatment of advanced and metastatic breast cancer.
K.-H. Altmann (
B
)
Department of Chemistry and Applied Biosciences,
Institute of Pharmaceutical Sciences, Swiss Federal Institute
of Technology (ETH) Zürich, HCI H405, Wolfgang-Pauli-Str. 10,
8093 Zürich, Switzerland
e-mail: karl-heinz.altmann@pharma.ethz.ch
F. Z. Gaugaz · R. Schiess
Department of Chemistry and Applied Biosciences,
Institute of Pharmaceutical Sciences, Swiss Federal Institute
of Technology (ETH) Zürich, Wolfgang-Pauli-Str. 10,
8093 Zürich, Switzerland
Keywords Cancer · Drug discovery · Epothilones ·
Microtubules · Natural products · SAR · Semisynthesis ·
Review
Introduction
Microtubule-interacting agents (MSA) are an important class
of antitumor agents [1], which are in use for the treatment of
a variety of cancers, either as single agents or as part of com-
bination chemotherapy [2,3]. MSA can be divided into two
distinct functional classes, namely compounds that inhibit
the assembly of soluble tubulin into microtubule polymers
(“tubulin polymerization inhibitors”) and those that promote
the assembly of tubulin heterodimers into microtubule poly-
mers and stabilize microtubules (“microtubule stabilizers”)
[4]. Among microtubule stabilizers the natural product taxol
(paclitaxel; Taxol
) and its semisynthetic analog docetaxel
(Taxotere
) (Fig. 1) are an important part of today’s arma-
mentarium for the pharmacotherapy of cancer [5].
After the elucidation of taxol’s mode of action in 1979
[6], it took more than a decade before other microtubule-
stabilizing agents with non-taxol-like structures were dis-
covered. Most prominent among these new microtubule
stabilizers are the epothilones, which are bacteria-derived
macrolides whose microtubule-stabilizing properties were
discovered in 1996 by a group at Merck Research Laborato-
ries [7]; the compounds themselves, however, had been first
isolated 9 years earlier from the myxobacterium Sorangium
cellulosum Sc 90 by Reichenbach and Höfle (Fig. 2)[8,9].
The major products originally isolated by Reichenbach
and Höfle were epothilone A and epothilone B (Epo A and B),
but numerous other members of this natural products family
have subsequently been obtained as minor components from
fermentations of myxobacteria [10]. Based on their unusual
123

384 Mol Divers (2011) 15:383–399
O
O
OH
OH
O
O
O
O
O
O
O
O
NH
OH
O
O
OH
OH
OH
O
O
O
O
O
O
O
NH
OH
O
O
Taxol (Paclitaxel; Taxol
®
) Docetaxel (Taxotere
®
)
Fig. 1 Molecular structures of taxol and docetaxel
O
OH
O
OH
O
N
S
O
R
13
9
11
12 13
15
17
21
26
20
19
R = H: Epothilone A
R = CH
3
: Epothilone B
Fig. 2 Molecular structures of epothilones A and B
NH
OH
O
OH
O
N
S
O
Ixabepilone (Ixempra )
®
Fig. 3 Molecular structure of the anticancer drug ixabepilone
mechanism of action (at the time of its discovery), Epo A and
B were quickly adopted as attractive targets for total synthe-
sis and, more importantly, as lead structures for anticancer
drug discovery.
Epothilone-based drug discovery research was addition-
ally triggered by the fact that epothilones, in contrast to
taxol, can inhibit the growth of multidrug-resistant cancer cell
lines at concentrations similar to or only slightly higher than
those required against drug-sensitive cancer cells, [7,1113]
including cells whose taxol resistance is mediated by spe-
cific tubulin mutations [13,14]. Epo B and a number of its
analogs have been demonstrated to possess potent in vivo
antitumor activity and at least seven epothilone-type com-
pounds have entered clinical evaluation in humans (although
several of these are not anymore under development). One
of these compounds, the epothilone B lactam BMS-247550,
was approved for clinical use in humans in 2007 (ixabepi-
lone, Ixempra
), (Fig. 3)[15].
As indicated above, epothilones have been attractive tar-
gets for total chemical synthesis and numerous syntheses of
Epo A and B have been successfully completed (for reviews
cf. [1622]). At the same time, the methodology developed
in the course of those studies has been exploited for the
synthesis of a host of synthetic analogs for SAR studies
(reviewed in [16,17,20,2327]), which highlights the differ-
ence in structural complexity (and, consequently, in synthetic
accessibility) between epothilone-type structures and taxol.
Beyond the investigation of fully synthetic analogs, however,
important aspects of the epothilone SAR (structure–activity
relationship) have also been derived from numerous semi-
synthetic epothilone analogs and ixabepilone (Fig. 3), the
only epothilone that has reached the approval stage so far, in
fact is a semisynthetic derivative of Epo B.
Semisynthetic derivatives of natural products hold a
prominent place in natural product-based drug discovery in
virtually all disease areas [28,29]; due to the structural
complexity of many biologically active natural products
[30], the chemical derivatization of material isolated from
natural sources often represents the only practical means
to explore structure–activity relationships and to produce
analogs with improved biological and/or pharmaceutical
properties. In cancer treatment, important natural product
derivatives include compounds such as etoposide or tenipo-
side (derived from podophyllotoxin) [3133], irinotecan and
topotecan (derived from camptothecin) [ 34 36 ], or doce-
taxel (derived from 10-deacetylbaccatin III) [5,37 ]. Even
for the natural product taxol [38], the sustained supply of
sufficient quantities of drug material for clinical use could
only be secured for some time through the development of a
semisynthetic production process from another natural prod-
uct, namely 10-deacetylbaccatin III [39,40]. Thus, it is not
surprising that semisynthetic approaches have also featured
prominently in the elucidation of the SAR of epothilones and
in the discovery of a number of clinical development candi-
dates. In fact, out of the seven epothilones that have entered
clinical evaluation in humans so far ( including the natural
product Epo B), only one is produced by total chemical syn-
thesis. This bias towards semisynthesis reflects the technical
(fewer chemical steps) and economic (cost of goods) advan-
tages still associated with natural product derivatization.
Obviously, the most fundamental provision for the genera-
tion of semisynthetic derivatives of a natural product is a suf-
ficient supply of the natural product starting material itself.
Fermentation processes are characterized by their own com-
plexities; thus, in the case of epothilones, only few groups
have had access to fermentatively produced starting materi-
als to perform semisynthetic work. Thus, most semisynthetic
epothilone derivatives reported in the literature originate
either from the Höfle group at the former “Gesellschaft für
Biotechnologische Forschung” in Braunschweig, Germany
(GBF, now “Helmholtz Centre for Infection Research”), one
of the discoverers of epothilones, or the group at Bristol–
Myers–Squibb (BMS), either by themselves or in collabo-
ration [8,41]. Semisynthetic work on epothilones, although
more limited in scope, has also been reported by groups at
Novartis, Kosan, and, most recently, our own group at the
ETH Zürich.
123

Mol Divers (2011) 15:383–399 385
This review will provide an overview on the semisynthetic
work that has been reported for epothilones in the public lit-
erature over the last 15 years. The discussion will address
both, the organic chemistry of the system, as well as the
most important aspects of the biological activity and SAR of
these derivatives, and it will be structured according to the
location of groups of modifications in the overall epothilone
structural framework. This will facilitate the comparison of
the biological effects of related structural changes. However,
it is important to note that biological data (i.e., tubulin poly-
merization, in vitro and in vivo antiproliferative activity) may
not always be directly comparable when originating from
different laboratories, due to differences in the experimental
conditions employed.
Semisynthesis and SAR studies
Modifications of the epoxide moiety
Modifications of the epoxide moiety have been an important
trait of the semisynthetic work on epothilones from the very
beginning, which is unsurprising in light of the multitude
of transformations that are conceivable for an oxirane ring
and the potential for further elaboration of the initial reaction
products. The earliest contributions to this area stem from the
GBF group and involved the transformation of epothilones
A, B, and C (vide infra) into a variety of C12/C13-modified
derivatives [23]. Thus, treatment of Epo A with HCl in THF
(aq) or with 1M HCl gave chlorohydrins 1 and 2 in 60–80%
overall yield in a ca. 2–4:1 ratio (in favor of the C12-chloro
isomer 1; Scheme 1)[42].
The corresponding bromo- and iodohydrins were obtained
with bromine or iodine in CCl
4
/CHCl
3
, respectively, with a
ca. 3:1 preference for the C12-halo regioisomer in both cases
[42]. Preferential (but not completely selective) opening of
the epoxide moiety at position 12 upon treatment of Epo A
with different nucleophiles (HCl, MgBr
2
· Et
2
O, NaI/TMS-I,
LiN
3
, Mg(OMe)
2
) has also been reported by the Novartis
group [43]; in contrast, the reaction of Epo A with MgBr
2
·
Et
2
OinCH
2
Cl
2
at 20
Cto5
C leads to the C13-bromo
isomer preferentially with less than 2% of the C12-regio-
isomer being formed [44,45](vide infra). Due to the greater
stability of the C12 over the C13 carbocation in S
N
1-type
reactions, treatment of Epo B with HCl gave chlorohydrin 7
as the only regioisomer in >80% yield (Scheme 2)[42].
Treatment of Epo A with a non-nucleophilic Brønsted acid
such as TFA led to rearranged products 3 and 4 exclusively
(85% combined yield), when acetone was used as the solvent
(Scheme 1)[42]. In contrast, exposure of Epo A or B to non-
nucleophilic acids in the presence of water gave diols 5/6
(Scheme 1) and 8 (Scheme 2), respectively. As for halohy-
drin formation, nucleophilic attack of the epoxide moiety in
Epo A occurs at position 12 preferentially, leading to isomer
O
OH
O
OH
O
N
S
OH
Cl
O
OH
O
OH
O
N
S
Cl
OH
O
OH
O
OH
O
N
S
OH
OH
O
OH
O
OH
O
N
S
O
O
O
O O
O
OH
OH
H
H
S
N
Epo A
1
b)
c)
2
+
a)
12
13
15R: 3
15S: 4
12S, 13S: 5
12R, 13R: 6
12S, 13S : 5a
12R, 13R: 6a
d)
15
Scheme 1 a THF/HCl (aq) or 1 M HCl, RT, 20 min, 60–80%, 1:2, 2:1–4:1. b 0.65 M CF
3
COOH, acetone, 50
C, 10 h, 50% (3) and 35% (4).
c 0.65 M CF
3
COOH, H
2
O, 23
C, 48 h, 55% (5) and 15% (6). dp-TsOH, acetone, 28% (5a) and 15% (6a). (Yields for 5a and 6a are from [43])
123

386 Mol Divers (2011) 15:383–399
O
OH
O
OH
O
N
S
OH
Cl
O
OH
O
OH
O
N
S
OH
OH
Epo B
7
a)
b)
8
Scheme 2 a THF/HCl (aq) or 1 M HCl, RT, 20 min, >80%. b CF
3
COOH/H
2
O, 23
CorH
2
SO
4
/H
2
O/THF, 60
C, 75% (H
2
SO
4
) or 45%
(CF
3
COOH)
O
OH
O
OH
O
N
S
O
OH
O
OH
O
N
S
OH
OH
O
OH
O OH
O
N
S
O
O
Epo C
a) b)
12
13
12R, 13S: 9
12S, 13R: 10
12R, 13S: 9a
12S, 13R: 10a
12
13
Scheme 3 a OsO
4
cat., NMO, t-BuOH, THF/H
2
O, RT, 75 min, 62%, 9:10, 2:1 (inseparable mixture). b acetone, p-TsOH, RT, 2 h, 90% (for
separable mixture of isomers)
5 as the major (but not the only) product; with Epo B diol
8 is the only isomer formed. The rearranged products 3 and
4 show substantially lower antiproliferative activity against
human cancer cells than Epo A [42].
The GBF group has also used OsO
4
-catalyzed dihydroxy-
lation of fermentatively produced Epo C (12,13-deoxyepo-
thilone A) to prepare cis-diols 9 and 10 (Scheme 3); these
compounds were subsequently converted into acetonides 9a
and 10a (as were diols 5 and 6; Scheme 1)[42](seealso
[43]). Acetonides 5a/6a and 9a/10a have been independently
reported by the Novartis group [43], which has also investi-
gated the biological activity of these analogs.
Interestingly, the acetonides derived from 13S diols 5
and 9 (i.e., 5a and 9a) proved to be only 10–15-fold less
potent antiproliferative agents than Epo A against the human
cervical carcinoma cell line KB-31 and its P-gp-expressing
KB-8511 subline (IC
50
values of 23 nM (10 nM) and 30 nM
(17 nM), respectively), while the respective diastereoisomers
6a and 10a were found to be 30–100-fold less potent [43];
likewise, Sefkow et al. [42] have reported 5a to have similar
antiproliferative activity as Epo C against the L929 mouse
fibroblast cell line. These data suggest that for a tetrahedral
geometry at C12 and C13 the size of the ring fused to the
C12–C13 single bond can be significantly increased with-
out substantial loss in biological potency (which does not
seem to be the case for analogs with a planar geometry of
the C12–C13 bond [43,46]). In addition, the data for 9a and
5a also illustrate that, given the proper absolute stereochem-
istry at C12 and C13, activity is retained even upon moving
from a cis-toatrans-fused system; this is in line with data
obtained for a number of synthetic C12,C13-trans epothil-
ones A [27]. It should be noted, however, that the absolute
configuration of compounds 5a and 9a (or the respective
diastereoisomers) has not been rigorously established in the
literature, and it is simply inferred from a comparison of the
biological data with those obtained for Epo A/epi-Epo A (the
inactive 12S,13R-isomer of Epo A) and 12S,13S/12R,13R-
trans-Epo A, respectively.
In contrast to the above acetonides, cis and trans diols 9
and 5 did not show any appreciable biological activity (IC
50
’s
for cancer cell growth inhibition >1 µM) [42,43]. Interest-
ingly, however, the azido alcohol obtained through epoxide
ring opening with NaN
3
at position C12 (i.e., (12R, 13S)-
12-azido-13-hydroxy-12,13-dihydro-Epo C (11), Scheme 4)
was found to be significantly more potent (e.g., IC
50
’s of
11 against the human cervix cancer cell lines KB-31 and
KB-8511 are 61 and 64 nM, respectively) [43]. This indi-
cates that the loss in activity for the above diols cannot be
simply ascribed to increased conformational flexibility. How-
ever, the interpretation of changes in cellular activity is not
straightforward, as they may be caused by a combination of
changes in target affinity, cellular penetration, and metabolic
stability.
Building on the above findings on the potent activity
of acetonides 5a and 9a, we have recently studied bicy-
clic epothilones 1315 (Scheme 4) and a series of related
analogs [48], in order to delineate the biological effects
of other 5-membered heterocycles fused to C12–C13 in
a non-planar arrangement and, in particular, to assess the
impact of substituents on the 5-membered ring. The synthe-
sis of Epo A-derived oxazolines 1315 was based on amino
alcohol 12 as the central intermediate (Scheme 4). As illus-
trated in Scheme 4, 12 was obtained through nucleophilic
ring-opening of the epoxide moiety in Epo A with azide anion
123

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TL;DR: This review describes only a handful of exemplary natural products and their derivatives as well as those that have served as elegant blueprints for the development of novel synthetic structures that are either currently in use or in clinical or preclinical trials together with some of their earlier analogs in some cases whose failure to proceed aided in the derivation of later compounds.
Abstract: Microbes from two of the three domains of life, the Prokarya, and Eukarya, continue to serve as rich sources of structurally complex chemical scaffolds that have proven to be essential for the development of anticancer therapeutics. This review describes only a handful of exemplary natural products and their derivatives as well as those that have served as elegant blueprints for the development of novel synthetic structures that are either currently in use or in clinical or preclinical trials together with some of their earlier analogs in some cases whose failure to proceed aided in the derivation of later compounds. In every case, a microbe has been either identified as the producer of secondary metabolites or speculated to be involved in the production via symbiotic associations. Finally, rapidly evolving next-generation sequencing technologies have led to the increasing availability of microbial genomes. Relevant examples of genome mining and genetic manipulation are discussed, demonstrating that we have only barely scratched the surface with regards to harnessing the potential of microbes as sources of new pharmaceutical leads/agents or biological probes.

67 citations


Cites background from "Diversity through semisynthesis: th..."

  • ...For further details on the epothilones and their mechanisms of action, see the latest review by Ferrandina and colleagues [64] in addition to those by Danishefsky [45] and Altmann and colleagues [6], which describe opportunities for the synthesis of epothilones and other derivatives....

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Journal ArticleDOI
TL;DR: The increasing number of natural MSAs indicates the potential discovery of more novel,natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

66 citations

References
More filters
Journal ArticleDOI
TL;DR: Several lactam analogues of the epothilones were prepared using a concise semisynthetic approach starting with the unprotected natural products and a novel regio- and stereoselective Pd(0)-catalyzed azidation reaction of a macrocyclic lactone resulted in satisfactory overall yields.
Abstract: Several lactam analogues of the epothilones were prepared using a concise semisynthetic approach starting with the unprotected natural products. Highlighted in this strategy is a novel regio- and stereoselective Pd(0)-catalyzed azidation reaction of a macrocyclic lactone. Subsequent reduction and macrolactamization of the resulting azide acid intermediates provided the desired macrolactams in satisfactory overall yields. The entire three-step sequence was streamlined into a “one-pot” process for the epothilone B-lactam, BMS-247550, which is currently undergoing phase I clinical trials. An initial total synthesis route to prepare the lactam analogue of epothilone C was completed and compared to the more direct semisynthesis approach. All of the lactam analogues were evaluated in vitro and the results are discussed.

128 citations


"Diversity through semisynthesis: th..." refers background or methods or result in this paper

  • ...While the situation is more complex in vivo and the stability of Epo B is much higher in human than in rodent plasma, their findings on the reduced activity of epothilones in the presence of mouse plasma have led the BMS group to pursue lactambased epothilone analogs as metabolically more stable alternatives to the natural macrolactones [41,59]....

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  • ...42 nM, respectively, for 34 and Epo B [59])....

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  • ...Using this chemistry, the BMS group has also developed an efficient one-pot process for the conversion of Epo B into lactam derivative 34, which involves the above Pd(0)-catalyzed ring-opening reaction, reduction of the azide with trimethyl phosphine and macrolactam formation with EDCI/HOBt [59]; this provides the desired lactam in 23% overall yield....

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  • ...These reports are in line with observations by the BMS group on the significantly diminished in vitro cytotoxicity of epothilones when preincubated with mouse plasma, which leads to rapid ester cleavage [59]....

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  • ...one order of magnitude lower than that of Epo B [23,59] (e....

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Journal ArticleDOI

127 citations


"Diversity through semisynthesis: th..." refers background or methods in this paper

  • ...Alternatively, reaction of E-64 with N-iodosuccinimide (NIS) gave the corresponding vinyl iodide, which underwent smooth Stille coupling with stannane 67 to give 68 [71,75]....

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  • ...While 59 could be converted to methylene derivative 60 in a modest overall yield of 15% (after deprotection), all attempts to re-introduce the natural thiazole side chain or to create a phenyl-based Epo A analog using Wittig-type chemistry were unsuccessful [47,71,75]....

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  • ...In fact, one of the epothilone-type clinical development compounds (ZKEpo, sagopilone) is an analog of Epo B with a benzothiazole side chain (analogous to the benzoxazole side chain in analog 81) [75]; however, sagopilone could not be prepared...

    [...]

  • ...While 64 was obtained as a 7/3 mixture of E/Z double bond isomers, these could be separated by preparative HPLC and the pure E-isomer was converted to the protected phenyl-based Epo A analog 66 through Suzuki coupling [71,75]....

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  • ...All analogs depicted in Scheme 16 were found to be significantly less active in proliferation or tubulin assays than Epo A [47,71,75]....

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Journal ArticleDOI
TL;DR: The correlation between binding, microtubule stabilization, and cytotoxicity of the compounds has been determined, and the binding constants correlate well with IC(50) values, demonstrating that affinity measurements are a useful tool for drug design.

121 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...[56], the replacement of the epoxide moiety by a cyclopropane ring also produces enhanced binding to stabilized microtubules in vitro....

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Journal ArticleDOI
TL;DR: The clarification of the mechanisms of action and resistance of topotecan and CPT-11 should enable us to understand their pharmacological behavior even better and might lead to the development of more potent camptothecin-derivatives in the future.
Abstract: Topotecan and irinotecan (CPT-11) are both anticancer agents active in the inhibition of topoisomerase I, an enzyme involved in DNA replication and RNA transcription. During the last decades, an immense amount of research into this class of anticancer agents has been conducted, the positive results of which led to the clinical use of topotecan and CPT-11 in ovarian cancer and colorectal cancer, respectively. Here, we review the currently most important pharmacologic aspects of these drugs, including their mechanisms of action, metabolism, activity- and toxicity-profiles and mechanisms of resistance, to provide a global insight into their pharmacology. We also discuss the effects of combinations with other anticancer agents, which have been tested for synergistic antitumor effects. Pharmacokinetic and pharmacodynamic biomodulation, to enhance the bioavailability of the active anticancer agent or to reduce drug related toxicities have currently reached clinical application. As pharmacogenetics enters the clinical stage, this will lead to more "fine-tuning" in anticancer treatment (for instance by individualized dosing). The clarification of the mechanisms of action and resistance of topotecan and CPT-11 should enable us to understand their pharmacological behavior even better and might lead to the development of more potent camptothecin-derivatives in the future.

116 citations

Frequently Asked Questions (15)
Q1. What have the authors contributed in "The chemistry and biological activity of semisynthetic epothilone derivatives" ?

In contrast, the current review provides a comprehensive overview on the chemical transformations that have been investigated for the major epothilones A and B as starting materials, and it discusses the biological activity of the resulting products. 

While 59 could be converted to methylene derivative 60 in a modest overall yield of 15% (after deprotection), all attempts to re-introduce the natural thiazole side chain or to create a phenyl-based Epo A analog using Wittig-type chemistry were unsuccessful [47,71,75]. 

The earliest contributions to this area stem from the GBF group and involved the transformation of epothilonesA, B, and C (vide infra) into a variety of C12/C13-modified derivatives [23]. 

Most prominent among these new microtubule stabilizers are the epothilones, which are bacteria-derived macrolides whose microtubule-stabilizing properties were discovered in 1996 by a group at Merck Research Laboratories [7]; the compounds themselves, however, had been first isolated 9 years earlier from the myxobacterium Sorangium cellulosum Sc 90 by Reichenbach and Höfle (Fig. 2) [8,9]. 

The resulting N-unsubstituted 12,13-aziridinyl-Epo A 21 has been converted into a series of N-substituted derivatives via alkylation, acylation, carbamoylation, or sulfonylation. 

While not accessible by base treatment and subsequent electrophilic quenching, C21-substituted epothilone derivatives can nevertheless be obtained through semisynthesis in a very efficient manner. 

Employing PLE-catalyzed hydrolysis of the lactone group and subsequent cleavage of the C12/C13 double bond by ozonolysis, the BMS group was able to establish the controlled degradation of Epo C into ester 82 (Scheme 20). 

Selective oxidation of the hydroxyl group on C3 in Epo A is more difficult and could only be accomplished in very moderate yield with a mixture of dimethylsulfide and dibenzoylperoxide [47]. 

The phenyl-substituted oxazoline 13 was found to inhibit human cancer cell growth in vitro with IC50 values around 20 nM [48]; thus, the activity of this analog is within a 10- fold range of the activity of Epo A and it is comparable with the activity of cyclic acetals 5a and 9a (vide supra). 

It is, therefore, unclear to what extent (if at all) the enhanced cellular activity of 14 (over 13) is a result of higher affinity interactions with the tubulin/microtubule system (possibly through H-bond formation between the pyridine nitrogen and a donor group on the protein). 

In addition to cyclopropyl-epothilones, the BMS group has also devised a strategy for the conversion of Epo A to a whole range of analogs incorporating a (substituted) aziridine ring in place of the epoxide moiety [45]. 

Bis-substitution of the oxygen-replacing carbon in cyclopropyl-Epo B by bulky bromine substituents leads to reduced activity, but the resulting analog is still more potent than Epo D against the HCT-116 cell line (IC50 against HCT116 of 3.8 nM versus 6.5 nM for Epo D [49]). 

The first implementation of this concept was reported by the GBF group, who used ring-opening olefin metathesis (ROM) with ethylene for the conversion of Epo C into the ring-opened product 69 in 73% yield (employing Grubbs II catalyst) [73] (Scheme 18). 

This bias towards semisynthesis reflects the technical (fewer chemical steps) and economic (cost of goods) advantages still associated with natural product derivatization. 

Epo B and a number of its analogs have been demonstrated to possess potent in vivo antitumor activity and at least seven epothilone-type compounds have entered clinical evaluation in humans (although several of these are not anymore under development).